In a rare statement to the newspaper, Mercer said, "I support President Trump and the platform upon which he was elected."
"My family and I have not communicated with Steve Bannon in many months and have provided no financial support to his political agenda, nor do we support his recent actions and statements," the daughter of hedge fund manager Robert Mercer told the Post.
The Breitbart News executive chairman irked Mercer, his top financial backer, when he told other donors that he thought he would have her financial support if he ran for president, the Post reported Wednesday, citing a person familiar with Bannon's conversations.
He also irked her with his actions in the Alabama Senate special election, the newspaper reported. Bannon supported Republican Roy Moore, the ex-judge who lost to Democrat Doug Jones in the deep-red state amid accusations of teen sexual abuse decades ago.
Bannon also alienated Mercer with remarks "that seemed to stoke unnecessary fights," the newspaper said. It did not specify what those remarks were.
The report followed the Wednesday revelation of Bannon's explosive comments to reporter Michael Wolff. Among other remarks, Bannon called Donald Trump Jr.'s June 2016 meeting with a Russian lawyer "treasonous."
The Bannon quotes from Wolff's upcoming book "Fire and Fury" prompted a remarkable statement from President Donald Trump, in which he declared that his former key advisor had "lost his mind."
On Thursday, White House press secretary Sarah Huckabee Sanders said Breitbart News should consider parting ways with Bannon.
Publisher Henry Holt and Company plans to release the book days early on Friday due to what it called high demand. The decision came after Trump's personal attorney sent a cease and desist letter to Wolff and Henry Holt and demanded an apology for the content of the book.
The Mercer family has funneled millions of dollars into Republican super PACs and Bannon's initiatives.
CNBC could not immediately reach Mercer for comment.